Threshold dependent voltage source

ABSTRACT

A circuit for generating voltages having values proportional to the threshold voltages (V T ) of n-channel transistors used in the circuit comprises a current mirror M 2 , M 3  having a reference current input generated from a reference voltage of value 2V t  by an n-channel transistor M 1 . The output reference voltage of value 2V T  by an n-channel transistor M 4  whose gate is coupled either to its drain, for output voltages greater than V T , or to the gate of transistor M 1  for output voltages less then V T .

BACKGROUND OF THE INVENTION

This invention relates to voltage sources and particularly to circuitswhich provide specific voltages which are dependent on the thresholdvoltage of transistors used in the circuit.

Such circuits are particularly useful in the field of CMOS IC's where itis advantageous to provide specific voltages whose values areproportional to the threshold voltage V_(T) of the transistors usedtherein. Such transistors may be either n- or p-channel field-effecttransistors. One application is in logic circuits where thresholdvoltage dependent voltages are required in order to switch thetransistors in the circuit so that logical decisions are made by thecircuit. Another application is in sensing amplifiers in which linesconnected to the inputs of the amplifier are precharged by voltagesproportional to the threshold voltage in order to improve thesensitivity of the amplifier.

SUMMARY OF THE INVENTION

Therefore it is an object of the invention to provide a circuit whichgenerates voltages whose values are proportional to the thresholdvoltage of the transistors used in the circuit.

Accordingly, the invention provides a voltage source circuit comprisinga current mirror having an input and an output and coupled to a firstreference potential line;

a reference current source coupled to the current mirror input orgenerating a reference current which is proportional to a thresholdvoltage; and

a bias transistor having a first current electrode coupled to thecurrent mirror output, a second current electrode coupled to a secondreference potential line and a control electrode coupled so as toproduce at its first current electrode a voltage dependent on thereference current,

wherein said current mirror output forms an output of the voltage sourcecircuit.

Preferably the reference current source comprises a transistor having afirst current electrode coupled to said current mirror input, a secondcurrent electrode coupled to said second reference potential line and acontrol electrode for receiving on input reference voltage.

As will be more fully described below, the control electrode of the biastransistor may be coupled to received either the input reference voltageor the voltage level at the current mirror output, depending on therequired output from the voltage source circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be more fully described by way of example withreference to the drawings of which:

FIGS. 1A and 1B show circuit diagrams of a basic embodiment of a voltagesource circuit according to the invention; and

FIGS. 2A and 2B show circuit diagrams of an improved embodiment of avoltage source circuit according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Thus, FIGS. 1A and 1B show circuit diagrams of a voltage source circuitproviding voltages which are dependent on the threshold voltage ofn-channel transistors. It comprises a current mirror composed ofp-channel transistors M₂ and M₃ each having one current electrodecoupled to a voltage supply line V_(DD). Transistor M₂ is diode-coupledwith its second current electrode coupled to its gate electrode which isalso coupled to the gate electrode of transistor M₃. The input to thecurrent mirror comprises the second current electrode of transistor M₂which is coupled to the first current electrode of an n-channeltransistor M₁. This transistor has its second current electrode coupledto a ground reference potential line and its gate electrode coupled toreceive an input reference voltage V_(REF).

In this embodiment of the voltage source circuit, the input referencevoltage V_(REF) is arranged to be twice the threshold V_(T) of then-channel transistors. Thus:

    V.sub.REF =2 V.sub.T                                       (0)

Since the current I through a transistor having a threshold voltageV_(T) and biased by a voltage V is described by

    I=K (V-V.sub.T).sup.2

where K is the transistor gain constant, the current through transistorM1 is

    I.sub.1 =K.sub.1 (2 V.sub.T -V.sub.T).sup.2 =K.sub.1 V.sub.T.sup.2(1)

This is the current input to the current mirror and the current outputfrom the mirror through transistor M₃ is:

    I.sub.3 =x I.sub.1 =x K.sub.1 V.sub.T.sup.2                (2)

where x is a constant determined by the geometry ratios of transistorsM₂ and M₃.

The output of the current mirror is coupled to the drain of an n-channelbias transistor M₄, this drain forming the output of the voltage sourcecircuit. The source of transistor M₄ is coupled to the ground referencepotential line and the gate of transistor M₄ is connected either to itsown drain (FIG. 1A) of the gate electrode of transistor M₁ (FIG. 1B)depending on the output voltage required from the voltage sourcecircuit.

If the gate electrode of transistor M₄ is coupled to its drain, as shownin FIG. 1A its drain source voltage V₄ is determined by:

    I.sub.3 =K.sub.4 (V.sub.4 -V.sub.T).sup.2                  (3)

Rearranging this, gives: ##EQU1## Substituting for I₃ from equation (2)gives: ##EQU2##

Thus the output voltage V₄ can be made to be any predetermined ratio ofV_(T) greater than one by appropriately choosing xK_(1/K).sbsb.4.

Similarly, if the gate electrode of transistor M₄ is coupled to the gateelectrode of transistor M₁ as shown in FIG. 1B, the transistor M₄ can bemade to operate in the triode region. In this case, the output voltageV₄ is given by: ##EQU3## Substituting for I₃ from equation (2) gives:

    V.sub.4.sup.2 -2 V.sub.T V.sub.4 +xK.sub.1 V.sub.T.sup.2.sub./K.sbsb.4 =0(7)

whose solution is: ##EQU4##

From this it can be seen that the output voltage V₄ can now be made tobe lower than the threshold voltage V_(T) by appropriate choices of x,K₁ and K₄.

Thus, by coupling the gate of transistor M₄ to the gate of thetransistor M₁, the ratio V_(4/V).sbsb.T is less than one and by couplingthe gate of transistor M₄ to the drain of transistor M₄, the ratioV_(4/V).sbsb.T is greater than one.

Although the above calculations were performed for V_(REF) =2V_(T), itwill be appreciated that a similar result will be obtained for V_(REF)being any value (n+1).V_(T). In this case:

    I.sub.1 =K.sub.1 ((n+1) V.sub.T -V.sub.T).sup.2 =K.sub.1 (nV.sub.T).sup.2(9)

so that for the gate of the transistor M₄ being coupled to its drain wehave, similarly to equations (2) and (3): ##EQU5## Thus:

    (V.sub.4 -V.sub.T).sup.2

giving: ##EQU6## so that ##EQU7##

To generate a current in transistor M₁, n must be greater than zero.However when V_(REF) is generated by diode-connected transistorsconnected in series, to realise ratios V_(REF/V).sbsb.T larger than twoi.e. three or four or more, requires higher values of the supply voltageV_(DD). Therefore a useful compromise is to set V_(REF) =2 V_(T).

One circuit in which a voltage V_(REF) with a value of approximately 2V_(T) is generated is shown in FIGS. 2A and 2B. In these Figurestransistors M₁ -M₄ are equivalent to those in FIGS. 1A and 1B,respectively and the output voltage is V₄. The reference voltage V_(REF)=V₁ is generated by resistor R and by transistors M₀₁, M₀₂, connected inseries between voltage supply line V_(DD) and reference potential line.However, the reference voltage V_(REF) will not be exactly 2 V_(T)because of transistors M₀₁ and M₀₂ which are diode-coupled, across whichthe voltage will be: ##EQU8## where I_(o) is the current through thetransistors M₀₁ and M₀₂ and K₀ is their gain constant.

However neither I₀ nor K₀ can be considered as having constant valuessince I₀ depends on the supply voltage V_(DD) and K₀ is a function ofprocess parameters and temperature. In the circuit of FIG. 1 andreferring to equation (0) the current I₃ controlled by voltage V₁ wouldbe: ##EQU9##

This current will be fed to transistor M₄.

To obtain a precise ratio of V_(4/V).sbsb.T equal to xK₁ V_(T) ² thecurrent I₃ must therefore be lowered by a value equal to: ##EQU10##

As shown in FIGS. 2A and 2B, a current of this value can be subtractedfrom I₃ using additional transistors M₅, M₆ and M₇. Transistors M₅ andM₇ are coupled in series between the ground reference potential line andthe output of the current mirror composed of transistors M₂ and M₃. Thegate of transistor M₅ is coupled the gate of transistor M₁ and the gateof transistor M₇ is coupled to the junction between transistors M₀₁ andM₀₂. Transistor M₆ is coupled between the ground reference potentialline and the input of the current mirror with its gate coupled to thegate of transistor M₇.

Transistor M₇ has a wide channel and acts as a voltage follower. Itsoutput voltage V₅ is given by: ##EQU11## The current I₅ throughtransistor M₅ operating in the triode region is: ##EQU12## which givesfrom equation (13): ##EQU13## By setting:

    K.sub.5 =2xK.sub.1

gives: ##EQU14##

Now subtracting I₅ from I₃ gives:

    I.sub.3 -I.sub.5 =xK.sub.1 (V.sub.T.sup.2 -2 I.sub.0/K.sbsb.0)(16)

This is close to the required value of xK₁ V_(T) ² but still requiresthe cancellation of the 2 I_(0/K).sbsb.0 term in order to achieve veryhigh precision for the ratio V_(4/V).sbsb.T.

This can be achieved by adding to current I₁ a current I₆ flowingthrough transistor M₆. By setting K₆ =2K₁ then:

    I.sub.4 =x [I.sub.1 +I.sub.6 ]-I.sub.5 =xK.sub.1 V.sub.T.sup.2(17)

Current I₄ flowing through transistor M₄ now has the required value andgenerates a voltage: ##EQU15## if its gate is connected to its drain asshown FIG. 2A or: ##EQU16## if its gate is connected to the gate of thetransistor M₁ as shown in FIG. 2B.

The above description refers to an embodiment of the circuit accordingto the invention in which voltages are generated whose value isproportional to the threshold voltage of the n-channel transistors. Togenerate voltages proportional to the threshold voltage of the p-channeltransistors a circuit complementary to that described above may be used.

I claim:
 1. A voltage source circuit comprising:a current mirror havingan input and an output and coupled to a first reference potential line;a reference current source coupled to the current mirror input forgenerating a reference current which is proportional to a thresholdvoltage; and a bias transistor having a first current electrode coupledto the current mirror output, a second current electrode coupled to asecond reference potential line and a control electrode coupled so as toproduce at its first current electrode a voltage dependent on thereference current, wherein said current mirror output forms an output ofthe voltage source circuit.
 2. A voltage source circuit according toclaim 1 wherein said reference current source comprises a transistorhaving a first current electrode coupled to said current mirror input, asecond current electrode coupled to said second reference potential lineand a control electrode for receiving an input reference voltage whichis proportional to the threshold voltage of said transistor of saidreference current source.
 3. A voltage source circuit according to claim2 wherein said input reference voltage has a value of substantiallytwice the threshold voltage of the transistor forming the referencecurrent source.
 4. A voltage source circuit according to either claim 2or claim 3 wherein the control electrode of said bias transistor iscoupled to receive said input reference voltage.
 5. A voltage sourcecircuit according to either claim 2 or claim 3 wherein the controlelectrode of said bias transistor is coupled to said current mirroroutput.
 6. A voltage source circuit according to claim 3 wherein saidinput reference voltage is produced at the gate electrode of a firstdiode-coupled transistor coupled via a second diode-coupled transistorto said second reference potential line.
 7. A voltage source circuitaccording to claim 6 further comprising means for adjusting the currentsat the input and output of the current mirror in order to correct thevoltage at the output of the voltage source circuit.
 8. A voltage sourcecircuit according to claim 7 wherein the adjusting means comprises afirst adjusting transistor coupled in series between said current mirroroutput and the first current electrode of a second adjusting transistor,the second adjusting transistor having a second current electrodecoupled to said second reference potential line, and a gate electrodecoupled to receive said input reference voltage and the gate electrodeof the first adjusting transistor being coupled to the gate electrode ofsaid second diode-coupled transistor, so as to subtract an adjustingcurrent from the current produced at the output of the current mirror.